At present, it is unknown of the origin of the One-Below-All, or when it came into being. What is known is that the "real" form of the One-Below-All is also unseen, other than those who witness it are destroyed.
It is also seemingly the archetypal dark opposite of the One-Above-All , through their relationship to each other appears to be minimal. At present, the being is trapped within the Below-Place, and with the dimensional Green Door blocking it from the rest of the multiverse.
The One-Below-All is the mystical third form (the third form itself being a concept rejected by Brian Banner's colleagues as well as others for lacking reliable proof) as well as the source of Gamma radiation, and has been implied to be the reason why it mutates certain individuals.
The One-Below-All was created by AI Ewing and Joe Bennett, first appearing in The Immortal Hulk #5 - In Every Mirror.
Major Story Arcs
In Every Mirror
The Green Door was first partly broken open when the first Gamma Bomb was detonated. This was also what allowed the One-Below-All to access the rest of the multiverse, through it required a host. When Brian Banner, already within Hell itself, was smashed until he reached the bottom of it (The Below-Place) The One-Below-All was able to take over his soul and body, and use it as a reliable host to interfere on Earth.
He first controlled Jailbait, a Gamma mutate that had been living with her boyfriend Hotshot: as a result, her powers were increased to a uncontrollable level, and Hotshot was forced to restrain her to make sure she didn't hurt herself or anyone else, later taking her to a church as he believed that she was being controlled by the devil, and needed a exorcism. She later died after the deity didn't require her body anymore, and killed her.
Sasquatch was the next host after being killed while in his human form. Fighting Hulk directly, he was able to fool Bruce into draining the Gamma energy out of Langkowski, allowing the One-Below-All to enter his mind directly via Gamma radiation. From there, the creature was able to influence his actions, and make him much more aggressive, in particular brutalising the Avengers.
When the Absorbing Man (made stronger due to a Bannerman transplant) drained the Hulk of most of his Gamma energy, he in turn also absorbed the essence of the One-Below-All as well, which allowed it to control his body as well. Having the energy and the host necessary to open the Green Door fully, Creel was forced to absorb the Gamma energy around the site of where the Gamma Bomb was launched, and was imploded, opening the Green Door and transforming all of Mexico into a dimensional expansion of the Below-Place.
While this process occurred, the Gamma energy absorbed from Hulk separated the two- Banner was trapped within the Green Door, his energy assisting in opening it, while Hulk was weakened, but had survived. The One-Below-All, empowered by the transformation, was able to take a semi physical form, revealing himself as a creature forming from clouds. It tried to kill Hulk by creating hundreds of demons to kill him, but the efforts of Absorbing Man in returning the Gamma energy to Hulk allowed him to defeat the horde, and dissipate the One-Below-All with a powerful thunderclap, forcing him back into the Green Door and reverting everything back to normal.
In a future that may occur, the One-Below-All was able to completely take over Banner's body and the Hulk himself, turning him into a unstoppable beast by infusing his powers with him. This Hulk was able to survive until the end of the universe itself, killing powerful individuals like Franklin Richards, Galactus , Mister Immortal, and Metatron and countless more until only it existed.
When the 9th universe came into being, The One-Below-All in the body of Hulk became known as "The Breaker of Worlds" or the "Breaker-Apart" which was a creature that destroyed all life in the universe. This host body was significantly larger than the average body of Hulk, and was considerably more powerful, being able to destroy planets simply by punching them and crushing them with its bare hands.
As of current, it is known if this future will or will not happen.
Powers & Abilities
As the source of Gamma mutation, The One-Below-All is directly responsible for all mutated beings that have ever been created through this method, and is also capable of not only controlling their bodies, but also massively enhancing their mutation to almost uncontrollable levels: turning Sasquatch into a bloodthirsty beast that was able to draw blood from Immortal Hulk or literally ripping apart the Absorbing Man are examples of this occuring.
While it has not been shown that the One-Below-All is capable of controlling beings not effected by Gamma radiation, it is perhaps true that it can manipulate them to a certain extent, for example making two individuals very aggressive and attack Sasquatch in human form with a knife for seemingly no reason- through subtle, it seems to be able to "push" people into doing things they would otherwise not do.
While in control of a host, the One-Below-All is also nearly impossible to remove. Even when asked directly by Doctor Strange, demons of immense power like Mephisto have openly stated that the deity is too strong to overcome, clearly showing superiority over others.
Based on what it may do in the future, the One-Below-All seems to be directly more powerful than all other cosmic beings in the universe, considering it was able to kill or destroy them all before the universe ended. This immense and highly unequal power may explain why it was sealed away in the Below-Place using the Green Door.
The One-Below-All can also create hundreds of demons from its dimension, as well as enslave the bodies and souls of those who pass within its dimension, turning them into husks that it can manipulate and twist to its own ends (like how it was able to use the soul of Rick Jones as A-Bomb, or General Ross as the Red Hulk to attack the Hulk with). These have been dubbed as "Qlippoth"- beings that gather strength from strength only.